Conventional filters for use in electrical circuits such as filters are typically constructed in LC circuits by using inductor and capacitor elements. In recent years, a miniaturization of circuits has been increasingly demanded, with the advancement of integrating technology. To cope with such a demand, an active filter which may be incorporated in such an integrated circuit has been developed. In order to construct this type of filter, a filter time constant must be set using a resistance and a capacitance. However, devices which define the time constant are not sufficiently precise in the interior of an IC. Therefore, an accuracy of the time constant is increased by making a current flowing through the devices, etc. be variable. As a circuit which is most effective for varying the current, there is known a voltage to current conversion circuit called as a Gilbert cell or a Gilbert circuit.
FIG. 1 shows a typical example of the variable voltage to current conversion circuit using the Gilbert cell.
In FIG. 1, differently related voltages V1 and V2 are supplied to input terminals 11 and 12, respectively. The input terminals 11 and 12 are connected to the bases of differentially paired transistors Q1 and Q2. The collector of the transistor Q1 is connected to power line Vcc, while its emitter is connected to the connection node of the collector and the base of a diode-fashion transistor Q3 and also to the base of a transistor Q5, through a resistor R11. The collector of the transistor Q2 is connected to the power line Vcc, while its emitter is connected to the connection node of the collector and the base of a diode-fashion transistor Q4 and also to the base of a transistor Q6, through a resistor R12. The emitters of the transistors Q3 and Q4 are grounded commonly through a current source I11. Also the common node of the emitters of the transistors Q5 ans Q6 is grounded through a current source I12. The collector of the transistor Q5 is connected to the power line Vcc. While the collector of the transistor Q6 is connected to the power line Vcc, and also to an output terminal 13.
In the variable voltage to current conversion circuit as described above, a differential input is given by Vin (Vin=V1-V2). A current flowing through the transistor Q1 is given by i1. A current flowing through the transistor Q2 is given by -i1. Further, a current flowing through the transistor Q5 is given by i2. A current flowing through the transistor Q6 is given by -i2. And a potential difference .DELTA.V between the bases of the transistors Q5 and Q6 will be in the relation shown by the following equation: ##EQU1##
In the equation (1), the Vt is a thermal voltage constant, the is a transistor saturation current having the same value over all transistors. Under the balanced state (.DELTA.V=0), currents i11 and i12 supplied from the current sources I11 and I12 differentially flow by each the 1/2 of the total currents. From the equation (1), the current i2 given as the following equation (2). EQU i2=(i12/i11)i1 (2)
On the other hand, when the equivalent emitter resistances of the transistors Q1, Q2, Q3 and Q4 are all given as re, the current i1 is obtained from the following equation. EQU i1=Vin/(r11+r12+4re) (3)
In the equation (3), the r11 and r12 represent the resistance of the resistors R11 and R12. Also the re represents the emitter resistance of the transistors. From the equations (1) and (2), the current i2 is given by the following equation. EQU i2=Vin/{(r11+r12+4re)(i11/i12)} (4)
This will become almost an output current. As seen from the equation (4), to vary the resistance component (r11+r12+4re)(i11/i12) constituting the filter time constant using a voltage to current converter, it is possible to change the current i12 of the current source I12.
In the circuit described above, the transistors Q3 through Q6 comprise a Gilbert cell and this Gilbert cell is ideally used as a convenient current converter as shown in equation (2). However, there this circuit experiences such large problems. For instance, as P-N junction logarithmic compressed voltage output by the transistors Q3 and Q4 was exponentially expanded by the transistors Q5 and Q6, it was largely affected by the characteristics of semiconductors. Thus mixing of noise is unavoidable and DC balance is bad. In particular, when composing an active filter, an S/N ratio is deteriorated largely as signal must go through this type of circuit, being adjusted by the amount of the circuits corresponding exponent. The smaller a signal level is, the more this phenomenon becomes disadvantageous. On the contrary, however, when a signal level is increased, distortion will increase as a gap between a logarithmic and exponential correction appears intensely.
When a signal is passed through a voltage to current converter which uses a Gilbert cell as described above, a S/N ratio is deteriorated, and a DC balance is deteriorated too.